The surface acoustic wave (SAW) devices have been used in many applications, such as resonators; replacements for inductance-capacitance (LC) filters; chemical or gas sensors; and radio frequency applications. SAW resonators are used in sensor applications because of their high sensitivity to surface mass perturbations based on the presence of a mass. The changes of the mass on the surface cause changes in phase velocity and center frequency of the waves. These characteristics of SAW resonators can be harnessed for use as biosensors, gas sensors and chemical sensors. S. L. Hietala et al., “Dual SAW sensor technique for determining mass and modulus changes,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 48, no. 1, pp. 262267, January 2001 and F. Bender et al., “Acoustic wave-based sensors using mode conversion in periodic gratings,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 46, no. 6, pp. 1497-1503, November 1999. The operating frequencies of SAW resonators depend on the width of electrodes; however the low insertion loss (<−6 dB) of common existing SAW filter designs limit them to being suitable primarily as radio frequency (RF) or narrowband filters.
In recent years, there has been much research in using SAW resonators as sensors. The applications of SAW resonators include chemical, thermal, pressure or biological areas. These devices sense differences by the interferences of acoustic waves on the piezoelectric surface. These interferences are from the change of total mass on the surface or the path of the transmission line. They cause a decrease in the velocity, center frequency and/or amplitude of acoustic waves. The time of acoustic waves' propagation between input and output transducers is also increased by these interferences. These changes can be sensed by the decrease of the center frequency, propagation loss or the increase of propagating time in the output transducer. A common design for a SAW sensor attracts the substance being measured so that the electrical signals collected at the output interdigital transducer (IDT) are different than the signals without the attracted substance.
Attention has been drawn in recent years to the use of SAW resonators in microfluidic devices. The acoustic waves are mechanical waves, which have motions in the Z direction on the surface. These surface motions typically have approximately 10 Å displacements and allow the SAW resonators to be used as microactuators. The acoustic waves can propagate into the fluid and become an inertial force. This mechanism to drive fluid actions allows the possibility for the SAW resonators being used in pumping, mixing or jetting devices of microfluidic devices.
SAW resonators have been used to design analog electrical filters and to substitute for inductance-capacitance (LC) filters in high-volume TV components, because they have very competitive performance for the price for use in intermediate-frequency (IF) circuit stages. Their operating frequency is typically in the range of 10 MHz to 1 GHz. There was intensive research done early on due to their wide range of applications; however, the SAW technology reached the limit of its low insertion loss (IL). The insertion loss of existing SAW filter designs is smaller than −6 dB (i.e., more negative or less than), which limits the utilization in the IF stages. For the receivers stage in telecommunications, the IF stages, which operate in millivolt signals, can tolerate the high loss of SAW filters. The RF filter circuits, which only have microvolt-level inputs, cannot use SAW filters due to the limitations of the signal-to-noise ratio and insertion loss. There are wider applications of low-loss SAW filters in RF front-end filters and antenna duplexers in communications.
The research and development of SAW resonators is related in the following three fields: (1) the geometry of the metal-film IDTs, (2) piezoelectric substrate, and (3) the wave propagation type. The insertion loss and frequency response characteristics can also be improved from these three factors. In general, the various SAW resonators for communication systems can be categorized into the four types discussed below. The frequencies for mobile and wireless applications typically range from 800 MHz to 2.4 GHz. The SAW filters are used in IF and RF stages in wireless communications. Bidirectional IDTs have uses as the delay lines for oscillators or equalizers, a PN-coded trapped-delay line for CDMA-TDMA systems, delay lines to reduce the multipath interference, fixed frequency reference oscillators, VCOs for first- or second-mixing stages in mobile transceivers, intermediate frequency (IF) filters for mobile receivers, and Nyquist filters. Resonator/filters have uses as fixed frequency and tunable oscillators, resonators for transmitter circuits, RF front-end filters for mobile systems, RF front-end filters for wireless receivers, and high-power antenna duplexers. Unidirectional IDTs have uses as low-loss IF filter for wireless circuits (>−3 dB), RF front-end filters for wireless communications, and multimode oscillators for spread-spectrum communications. Nonlinear operation SAW resonators have use as convolvers for spread-spectrum communications.